The invention relates to a tire with radial carcass reinforcement, intended for equipping a heavy vehicle such as a transport vehicle or construction machinery. It refers in particular to a tire having an axial width greater than 37 inches.
A tire of this kind, usually intended to bear heavy loads, comprises a radial carcass reinforcement and a crown reinforcement composed of at least two working crown plies which are formed by non-extensible reinforcement elements that cross from one ply to the next, forming equal or unequal angles of between 10° and 45° with the circumferential direction.
Crown reinforcements of radial tires, in particular as regards very large tires, undergo pronounced deformations which give rise to longitudinal and transverse shear stresses between the edges of two crossed plies (the longitudinal shear is greater than the transverse shear when the cords of crossed plies are at small angles with the circumferential direction) at the same time as a delaminating stress, a radial stress tending to separate the edges of two plies radially from one another. The said stresses are primarily due to the inflation pressure of the tire, which has the effect that the so-called belt pressure between the carcass reinforcement and the crown reinforcement tends to cause the said crown reinforcement to expand circumferentially. The said stresses are moreover due to the load borne by the tire during travel, once a contact surface is produced between the ground and the tire. The said stresses are, furthermore, due to the slip angle of the tire during travel. These stresses produce cracks in the rubber mix adjoining the end of the shortest ply, and these cracks are propagated through the said mix, having an adverse effect on the service life of a crown reinforcement and hence of the tire.
A clear improvement to the service life has been achieved by using in the crown reinforcement at least one protective crown ply having an axial width greater than the width of the axially widest working ply.
The term “axial” means a direction parallel to the axis of rotation of the tire, and the term “radial” means a direction intersecting the axis of rotation of the tire and perpendicular thereto. The axis of rotation of the tire is the axis about which it turns in normal use. The circumferential center plane is a plane perpendicular to the axis of rotation of the tire, dividing the tire into two halves. A radial plane is a plane containing the axis of rotation of the tire.
Another solution, as described in the patent FR 2 421 742, consists in more favorable distribution of the stresses which give rise to separation between working crown plies and which result from the slip angle of the tire, by multiplying the number of working plies.
Multiplying the working plies is not without disadvantages, in particular at the center of the crown reinforcement, where the number of plies has a direct influence on the stiffness in flexure of the crown of the tire. When this stiffness increases, the result is that impacts on the crown of the tire, such as in particular on passing over large stones, may lead to irreparable damage to the tire as a result of this increased stiffness.
Patent application WO 00/54992 has moreover proposed, as a way of avoiding this disadvantage, making a working crown reinforcement composed of at least three continuous working plies and at least one half-ply on either side of the circumferential center plane, disposed between the edges of at least two radially adjacent continuous working plies, and whereof the special feature is in particular the fact of forming an angle with the circumferential direction greater than 25° and greater by an amount of between 5° and 15° than the largest of the angles of the continuous working plies. The results obtained with this type of structure have been perfectly satisfactory for the sizes of tire tested.